A pulsed load or switched load is a load which requires a large supply of power intermittently. A switching mode power supply for such a load includes a switch that operates at a low frequency to allow high amplitude current to be supplied intermittently to the load. The switch “chops” a low voltage input DC source, thus converting it to a high-ripple pulsed AC voltage, which is transformed to an interface voltage that may then be used directly or rectified and smoothed to produce DC voltage, according to the application. Ripple current refers to the variation in the DC signal. Owing to the sharp changes in the current requirement when the switch is on or off, the ripple current in the current supplied by the main power supply can cause high radio frequency interference RFI which is reflected on to the main supply source if it is not suppressed.
The circuit that converts source input voltage DC to pulsed AC is known as a switching converter or simply ‘converter’ of which there are two principal types, ‘Buck’ and ‘Boost’ although there are several hybrids and variations. The Buck converter normally converts the voltage down so that the output voltage of the converter is lower than the input voltage to the converter by a factor δ that is equal to the duty cycle of the switch. Duty cycle is the ratio between the duration during each cycle that the switch is ON to the total time between successive pulses, i.e. the period, i.e.
                              V          OUT                =                ⁢                  δ          ·                      V            IN                                                  δ        =                ⁢                                            T              ON                        T                    =                                    T              ON                                      (                                                T                  ON                                +                                  T                  OFF                                            )                                          where:
VIN=input voltage;
VOUT=input voltage;
δ=Duty cycle
TON=Time when switch is ON
TOFF=Time when switch is OFF
T=Pulse period=(TON+TOFF)
The Boost converter converts the voltage up so that the output voltage of the converter is higher than the input voltage to the converter by a factor
      1          1      -      δ        ,where δ is equal to the duty cycle of the switch. Since δ is less than 1, the factor
  1      1    -    δ  is greater than 1.
It thus emerges from the foregoing discussion that regardless of the type of converter that is employed, the output voltage of the converter is a function of the duty cycle of the switch. This allows accurate regulation of the voltage simply by controlling the duty cycle of the switch voltage, and this is easily achieved using pulse width modulation, PWM to control the pulse width during which the switching voltage pulse is ON. Since the period of the switching voltage pulse remains constant, adjusting the pulse width of the ON time varies the duty cycle of the switching voltage.
In the related art, an RFI filter at the input of the power supply is used to filter the radio frequency interference so that RFI is not reflected on to the main supply source. Maintaining the ripple current as low as possible also diminishes the conduction losses related to high root mean square (RMS) current values, which reduce the current delivery capability of the supply source. However, when a switch power supply is used in conventional circuits for supplying power as intermittent voltage bursts, the sudden voltage burst reflects on the line causing sudden and intermittent voltage reductions on the line. When very high power bursts are being supplied, the RFI filter becomes bulky and expensive.
It would therefore be desirable to provide a power supply for a switched load which allows the RFI filter to be reduced in size or even eliminated altogether, particularly when high power is being switched.